Andromeda 5x bigger than previously thought.

I have a question about the article. What do current galaxy formation theories say about why the cores of galaxies like Andromeda have more metal-rich stars than their halos do? I am guessing that the halo stars slowly get drawn into Andromeda's central black hole and as a result, they coalesce into stars that are then capable of the fusion reactions and eventually the supernovae that are capable of creating metals, is this close?